Over the past decade there have been increasing demands to lower the cost of space transport to geostationary and other orbits as well as perform missions which are simply not possible with existing launchers such as manned exploration of the moon and Mars. Implicit is the demand that reliability be enhanced and certainly not degraded. Unspoken but also implicit is that commercially viable launchers must fill this broad range of demands since single-purpose launchers such as the Saturn rocket or Space Shuttle are cost prohibitive, even for governments with substantial space budgets. The commercial launchers presently being used for most missions are the result of decades of evolution and have become highly refined and proven. Each individual system on the launchers has been raised to a high level of performance which is very difficult to improve upon, even with large investments in engineering development. Since only incremental improvements can be expected by addressing individual systems, there is a need to view space vehicle systems in a more broad sense to determine if substantial improvements can be accomplished.
One example of a high performance, highly evolved upper stage is the Centaur®. The Centaur® upper stage is capable of delivering payloads to nearly any orbit from Low earth Orbit to interplanetary trajectories. The Centaur® is capable of delivering a high energy to the payload by burning liquid hydrogen (LH2) and liquid oxygen (LO2) in a very high efficiency, low weight engine such as the RL10. The total mass of the Centaur when empty is less than 2.5 mt, yet it can contain in excess 20 mt of propellant. Propellants are stored in lightweight stainless steel tanks whose structural rigidity is provided primarily by the pressure of the propellants within the tank. In order to keep the tanks from collapsing prior to the loading of propellant, the tanks are pressurized with gas. In the Centaur®, a common double bulkhead is used to separate the LO2 and LH2 tanks. The two stainless steel containers are separated by a very thin layer of insulator which is contained within a hermetic cavity. Therefore, the appearance is of a single tank, but it is divided into the separate LO2 and LH2 tanks with an intermediate vacuum cavity. The extreme cold of the liquid hydrogen on one side of the bulk head creates a vacuum within the intermediate cavity. The insulator prevents the two metal bulkheads from contacting thereby maintaining a low thermal conductivity, thus preventing heat transfer from the comparatively warm LO2 to the super cold LH2. The exterior of the tanks are also insulated to suppress heat flows from the external environment to the propellants.
While on the ground and filled with propellants, the tank pressures are controlled by valving which maintains the tanks within a specific pressure band. The propellants within the tanks boil due to external heating and the vapor formed is passed through these regulating valves which hold the tank internal pressure within band regardless of the heating and attendant boil off vapor mass flow. By controlling the tank pressure at which the propellants boil, their saturation conditions are established. For the sake of maximizing the density of the propellants and hence reducing the size of the vehicle tanks, the pressures and temperatures are kept as low as possible within the tanks. These vent valves are thus precision cryogenic regulators that are complex, costly and heavy.
While on the ground, loads imparted to the vehicle are quite low, and the stiffening effects of the low internal pressures controlled by vent valves are sufficient to maintain structural integrity of the vehicle. However, during the ascent phase of flight and also prior to operation of the upper stage engines, the pressures within the vehicle tanks must be raised. In the case of ascent, the vehicle must be further stiffened so that it can survive the very high bending and compressive loads generated by aerodynamic, thrust and inertial effects. Pressures are raised prior to engine start to permit the proper operation of the engine pumps. These high capacity pumps must receive propellants whose pressure is substantially above their saturation pressure. This saturation pressure was effectively set prior to liftoff by the valving controlling tank pressures. Without system pressure maintained above saturation pressure, the propellants would boil within the pumps and they would cease to function properly. This margin is commonly referred to as Net Positive Suction Pressure (NPSP) and is commonly on the order of 3-10 psi.
In most modern upper stage vehicles, these pressurization demands are met by introducing gaseous helium into the ullage spaces of the propellant tanks. This helium is stored in separate vessels, typically at high pressure, and is delivered via valves to the propellant tanks at need. Helium is used since it has a low density, is chemically inert, and does not condense to a liquid at the cryogenic temperatures seen in the LO2 and LH2 tanks. Hence it can be used to pressurize both the LO2 and LH2 tanks with a tolerable mass penalty. Once the upper stage engines are operating, it is possible to perform the pressurization task by bleeding small amounts of warm H2 and O2 gases from the engine. This reduces the amount of helium required for the mission. The amount of helium required is thus dictated by the size of the propellant tanks, their pressure and the number of burns which are expected to be performed. The mass of the hardware required to contain this helium is very significant and many approaches have been taken to suppress system complexity and weight. However even the most advanced existing systems have strict limitations on their capabilities. These systems all have a limited amount of GHe and hence the number of engine burns, tank size, and other factors are all limited. Even a small leak of helium from the storage systems can result in a catastrophic loss of pressurant and hence mission failure.
During flight the upper stage propellant tanks will continue to absorb energy from the environment, albeit at a lower rate than what was present prior to launch. During engine burns, elevated tank pressures are maintained with GHe, gaseous O2 or H2 to establish and maintain sufficient NPSP and hence will end up at the end of a burn at a tank pressure above the saturation condition of the propellants. As heat is applied to the liquid propellants, they will gradually increase in temperature until their saturation pressure matches the partial pressure of H2 or O2 in the ullage gas. At this point, the propellants begin to boil. Tank pressures rise as the boil-off continues. If no action is taken prior to the next start of the engines, the system must be pressurized above this new higher pressure. The incremental increases in tank pressures therefore directly drive the peak operating pressures of the tanks, and hence their mass. Therefore, tank designs may have to account for much higher pressures, such as a 60 psia capability, which results in a substantial mass penalty.
To mitigate this pressure building effect on missions lasting more than a few minutes, it is common to vent the pressure in the ullage space to a level close to the original saturation pressure. Especially on the LH2 tank, during a mission to geostationary orbit, this venting may be performed multiple times. The energy absorbed from the environment is stored in the enthalpy of the ullage gases which therefore must be subsequently dumped overboard.
A significant limiting factor for all missions in space is accounting for the ullage losses associated with the continual boil off of cryogens. It is this propellant loss that has prevented to date the use of cryogenic propulsion systems for missions to the moon or indeed any mission with a duration that is much longer than one day. One of the most effective approaches for reducing losses is to apply a very low thrust to settle the propellants within the tanks to fixed locations, generally towards the aft end of their respective tanks. Less than a thousandth of one G is required to achieve this effect. Settling thrust segregates the liquid and gaseous phases of each propellant. Cold liquid propellant is thus physically separated from much warmer gas by the settling thrust. The quiescent gaseous ullage, in a microgravity environment without significant convection, then behaves as an excellent insulator and blocks heat from entering the liquid propellant surfaces. Heat is conducted down the side walls from the warm ullage side of the tank to the cold liquid side but this is inhibited by the relatively long conductive distances, reduced thermal conductivity due to the cryogenic operating temperatures and low wall thicknesses. Naturally the thinner these walls are the better. Thus it can be seen that a tank with a low gage and hence low allowable operating pressure is also thermally superior. All of these effects conspire to slow boil off when settling is imposed.
Settling thrust is typically provided by one or more small rockets fueled by hydrazine. On the Saturn S-IVB stage, the ullage gases were burned in a small thruster to maintain vehicle settling and some of the heat of the burning H2 and O2 was used to warm cold helium up for use in the pressurization system. Other vehicles such as the Delta Cryogenic Second Stage simply vent the boil-off gas aft to produce a small amount of settling thrust during long duration missions. Most often though, these waste ullage gases are simply dumped. Depending on vehicle design and mission duration, these wasted propellants can weigh into the hundreds of pounds. Naturally, the amount of time that a hydrazine system can support settling is strictly limited by the amount of propellant that it contains. Despite the best conservation efforts, hydrazine-based settling can at best be sustained for a handful of hours. Once settling is lost the surface tension effects within the propellants will gradually cause the interior of the propellant tanks to be fully wetted, temperature segregation will be lost and boil off rates can triple.
The vehicle must also provide a means for changing its attitude, and this function is also typically done with a hydrazine fueled thruster system. On the Centaur® vehicle, the attitude control thrusters and settling thrusters share a common supply system. While the settling function consumes the vast majority of hydrazine capacity, the attitude control task cannot be ignored. Settling thrusters can be commanded off, but the vehicle attitude must be stabilized for various reasons to include (i) maintaining radio links to the ground, (ii) providing an optimal attitude relative to the sun so that components such as avionics do not get too hot or cold, and (ii) suppressing heating of the main propellant tanks. Even if settling is eliminated, the attitude control function alone can consume hundreds of pounds of propellant over the course of a multiday mission. This propellant requirement is insupportable by most commercial upper stage launch vehicles.
Regarding the use of hydrazine as a propellant, while its application to space vehicles is widespread, there are a number of problems associated with its use. Hydrazine is a highly toxic, highly corrosive fluid that is compatible with only a handful of materials. Handling hydrazine requires hazardous procedure precautions, often requiring the use of positive-internal pressure inflatable SCAPE (Self Contained Atmospheric Protective Ensemble) suits to protect technicians loading a vehicle. Hydrazine can only be used in a narrow band of temperatures near room temperature. Hence, elaborate thermal control measures including heaters are mandatory, thus burdening the electrical storage system and exacerbating propellant heating. Hydrazine is also quite costly. Hydrazine is also a very inefficient fuel, delivering only a miserly specific impulse of 235 seconds. The advantages of Hydrazine are that as a fuel, it is simple and reliable to use assuming the appropriate environmental conditions can be maintained during its storage and delivery to a reaction chamber. Hydrazine is catalytically decomposed in a simple reaction chamber and does not require an ignition system or even an oxidizer. Nevertheless, the continued use of hydrazine sets harsh boundaries on improving overall vehicle operations and costs.
The electrical systems on the upper stage currently use large electrochemical batteries to provide power. This battery technology has evolved over decades to favor batteries of increasing power density and attendant sophistication. The desire to provide redundancy has doubled the storage demand. Even with the best modern technology, these batteries are extremely heavy, costly and can only supply enough power for less than a day's operation of a vehicle such as a Centaur®. Without a means to recharge these batteries, they set a strict limit on mission duration. Unfortunately the two common sources of power for recharge are solar panels and fuel cells, and these systems are both very costly to incorporate on a vehicle. Use of solar panels requires vehicle orientation control relative to the sun, and are physically bulky with complex deployment mechanisms. Most spacecraft that use solar panels are effectively in zero-G conditions, and hence large deployed solar panels are never exposed to high loads. A vehicle like Centaur® will generate acceleration forces in excess of 2 G's, and hence the mounting system for even a small solar array would be very heavy.
Fuel cells, while being more compact than batteries efficient and seemingly simple, are quite costly and complex to operate and support due to their intolerance of inert gases within the reactant streams and due to the necessity to dispose of the water they produce. To date, only manned vehicles such as the space shuttle can justify their cost and complexity.
While cost reduction, increased simplicity and reliability are primary goals in an improved vehicle, there is also an increasing need to expand mission capabilities beyond merely moving heavier payloads. Current missions are performed over a maximum flight duration of less than a day. However if the vehicle could efficiently fly for longer, it would be extremely valuable. Missions such as those to the moon require coast durations measured in days. The increasing amount of space junk in orbit will soon require the deliberate disposal of not only obsolete satellites but also the stages which placed them in orbit. This disposal activity at present would impose large performance penalties which would drastically increase the cost to orbit. However by performing disposal maneuvers at optimal times, this function can be accomplished with a minimum of cost. Missions such as space junk removal require a vehicle be capable of flying for days to weeks. In summary if one wishes to improve vehicle system performance and cost, yet expand the mission duration and improve reliability, a broader view of the vehicle must be taken to include a simultaneous analysis of vehicle thermodynamics, power, propellant and pressurant storage limitations, vehicle structural and thermal interactions, and the demands of widely varying missions. While it may be possible to redesign vehicle systems on a micro level, that is, to redesign selected systems based on specific mission requirements, this design approach inevitably compromises the majority of missions and can also create a proliferation of system designs that are all slightly different and likely incompatible. This micro level design solution is the origin of the present state of most space vehicle capabilities.
The use of waste ullage gas was recognized in the 1960's as a potential source of fuel for an auxiliary engine on the Saturn S-IVB. NASA recognized that these ullage gases could be captured and reused within an internal combustion engine that could be used to provide power for the upper stage vehicle. Although this recycling or reuse of the ullage gases was recognized development stalled with the proof of concept of a H2/O2 burning internal combustion engine. The concept was never flown.
There are a number of examples of improvements made to rocket propulsion systems in order to increase main engine propulsion efficiency, or to simplify the components of a launch vehicle, with one intent being increasing the available payload of the vehicle.
One example of such a reference is the U.S. Pat. No. 5,282,357 for a high-performance dual-mode integral propulsion system. This reference discloses a propulsion system in which pure hydrazine is used as the fuel for both a bi-propellant rocket engine for high thrust performance and in multiple mono-propellant thrusters for station keeping and attitude control functions. The use of the common fuel for both modes of operation significantly reduces propellant weight and inert propulsion system weight. For station keeping, the mono propellant thrusters can be augmented in performance by employing either electrothermal or additional direct chemical energy, arc jet operation, or force fuel acceleration to provide increased specific impulse values.
The U.S. Pat. No. 6,135,393 provides a spacecraft attitude and velocity control thruster system that incorporates mono-propellant RCS thrusters for attitude control and bi-propellant scat thrusters for velocity control. Both sets of thrusters are designed to use the same liquid fuel, supplied by a pressurized non-pressure regulated tank, and operate in a blow down mode. In the propulsion system, such station keeping and attitude control thrusters may function in conjunction with a large thrust apogee kick engine that uses the same propellant fuel. Hydrazine and bi-nitrogen tetroxide are preferred as the fuel and oxidizer, respectfully.
Despite improvements in general rocket technology, to include increasing the efficiencies of rocket engines and components, there is still a need to provide even greater efficiencies, and to simplify space launch vehicle systems while carrying larger payloads over longer durations.